The ATM protocol is designed to carry a variety of media types in fixed-length cells with a high level of quality control. Virtual circuits are established across ATM connections to transfer particular flows from one ATM end station to another ATM end station. A VC is defined by a virtual channel (identified by a virtual channel identifier, VCI) and a virtual path (identified by a virtual path identifier, VPI) that exists between the end stations.
FIG. 1 depicts a virtual circuit (identified by a VPI/VCI combination) that is established over an ATM link that includes three ATM nodes. As shown in FIG. 1, the virtual channel extends from a first ATM enabled router 104 (router 1) through an ATM switch 106 and to a second ATM enabled router 108 (router 2).
In some network topologies, two ATM nodes may be connected by an ATM enabled Ethernet switch. For example, FIG. 2 depicts two ATM enabled routers 204 and 208 that are connected by an Ethernet switch 206 that is equipped with ATM interfaces 210 and 212. In the example, a first ATM connection is established between the first ATM enabled router and the Ethernet switch and a second ATM connection is established between the Ethernet switch and the second ATM enabled router. The ATM interfaces of the Ethernet switch act as end stations of the ATM links and therefore two VCs are needed to transfer ATM cells from the same VC between the two routers. Specifically, the first VC (VC 1) is terminated at the input to the Ethernet switch and the second VC (VC 2) is established at the output of the Ethernet switch. It would be ideal to continuously carry a single VC from the first ATM enabled router (router 1) to the second ATM enabled router (router 2), however because of the differences between ATM and Ethernet, the first ATM connection and VC are terminated at the input to the Ethernet switch and the second ATM connection and VC are established at the output port of the Ethernet switch.
One technique for mapping the second VC at the output port of the Ethernet switch involves mapping each incoming VC to a virtual local area network (VLAN) at the input port of the Ethernet switch, switching the ATM traffic from each VC through the Ethernet switch via a VLAN, and then mapping each VLAN to the proper VC at the output port of the Ethernet switch. Although this technique works well, the field for identifying VLANs in an Ethernet network is limited by the IEEE 802.1Q protocol to 12 bits, or 4,096, unique VLANs. When this technique is used to aggregate a large number of subscriber units, such as DSL modems, the small number of unique VLANs becomes the limiting factor in the scalability of the network. In addition to the scalability problem, mapping each VLAN to its proper VC at the output port of the Ethernet switch requires an additional look-up, which is a slow process that could reduce the performance of the ATM interfaces. There are also limitations on the number of learned MAC addresses that can be stored in memory. Similar problems exist when data segments from protocols, such as Frame Relay, are transferred across an Ethernet switch.
In view of the limitations involved with using VLANs to transfer data segments, such as ATM cells or Frame Relay frames, and VC information related to the data segments across an Ethernet switch, what is needed is a method and system for rapidly transferring non-Ethernet data segments and VC information across an Ethernet switch that does not require additional learning or look-ups and that scales beyond 4,096 unique identifiers.